Superconductivity and Magnetic Properties of high-quality single crystals of $A_x$Fe${}_2$Se${}_2$ ($A$ = K and Cs)

Abstract

We successfully grew the high-quality single crystals of $A_x$Fe${}_2$Se${}_2$ ($A$ = K and Cs) by self-flux method. Sharp superconducting transition was observed for both types of crystals. The crystals show the onset superconducting transition temperatures ($T_{\mathrm{c}}$) of 31 K and 30 K for K- and Cs-compounds, respectively, with nearly 100% shielding fraction. The crystals show quite high resistivity in the normal state of more than 160 m$\Omega$ cm and 1300 m$\Omega$ cm maximum resistivity for $K_{0.86}F{e}_{2}S{e}_{1.82}$ and $C{s}_{0.86}F{e}_{1.66}S{e}_2$ single crystals, respectively. Much larger upper critical field $H_{c2}$ is inferred from low-temperature iso-magnetic-field magnetoresistance in these crystals than in FeSe. The anisotropy $H_{c2}^{ab}$(0)/$H_{c2}^c$(0) is around 3 for both of the two materials. Anisotropic peculiar magnetic behavior in normal state has been found for $C{s}_{0.86}F{e}_{1.66}S{e}_2$

5 pages, 5 figs

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